I was actually really happy with this. The characters are perfect - I could practically hear their voices saying the lines as I read them. The art is slightly different to the animated series, which is to be expected, but otherwise I thought it worked well.
The plot is pretty much what you would expect to happen after the events of the TV series - Aang and Zuko's mission of bringing peace between groups that have been killing each other for a century was never going to be easy. The three chapters are essentially the equivalent of maybe a season of the show, but very condensed, and with a lot less filler scenes and side-plots.
And to the comment below about Zuko's mother not being mentioned: she comes up a lot in the third chapter, and it looks like that will be resolved in the next tie-in series - The Search coming out in March 2013.
The only issue I had with this was the romance between Aang and Katara. The majority of the scenes where they interact are lovely - subtle, and yet clearly demonstrating their affection for each other. So then when they randomly start to call each other 'Sweetie' and the artist draws hearts all around the panels as they stare into each other's eyes, it seems both out of character, and unnecessary.
Otherwise, very worth buying.